World Wide Study Bible
a Bible passage
21. Who knoweth—Not doubt of the destination of man's spirit (Ec 12:7); but "how few, by reason of the outward mortality to which man is as liable as the beast and which is the ground of the skeptic's argument, comprehend the wide difference between man and the beast" (Isa 53:1). The Hebrew expresses the difference strongly, "The spirit of man that ascends, it belongeth to on high; but the spirit of the beast that descends, it belongeth to below, even to the earth." Their destinations and proper element differ utterly [Weiss].